1. Field of the Invention
The present invention relates to a coordinate detecting apparatus used as an input unit of a computer and, more particularly, to a circuit board for an electromagnetic induction type coordinate detecting apparatus and a coordinate detecting apparatus employing the same.
2. Description of the Related Art
A conventional coordinate detecting apparatus, generally called a "digitizer tablet", has a surface for detecting coordinates. When an operator points to a position on the coordinate detecting surface with a position pointing device, the apparatus detects the position pointed to, generates coordinate data corresponding to the position, and sends the data to an external unit such as a computer.
Such a coordinate detecting apparatus requires substantial reduction of noise which is produced by the processing circuit thereof because of two main reasons. Firstly, such noise may adversely affect other electronic apparatus or the like. Secondly, noise produced by the processing circuit may adversely affect the sensor region of the coordinate detecting apparatus and thus impede enhancement of the performance characteristics of the apparatus such as precision.
Reduction of electromagnetic wave noise is particularly critical in an electromagnetic induction type coordinate detecting apparatus, because this type of detecting apparatus detects the position pointed to by a position pointing device, by detecting alternating field.
The circuit board of an electromagnetic induction type coordinate detecting apparatus can be considered to consist of a sensor region and a processing circuit. The sensor region means a region of the circuit board which is provided corresponding to an effective area of the coordinate detection surface (an area in which reliable performance, such as a certain level of coordinate detection precision, is obtained), for example, a region in which a loop coil is composed of a group of sensor lines which are arranged along X axis and Y axis. The processing circuit includes: a circuit for supplying power to the sensor region; a circuit for coordinate calculation; and a controlling circuit. The sensor region and the processing circuit may be formed together on a single printed circuit board or separately on different printed circuit boards.
If at least the processing circuit of a circuit board is composed of a multi-layer board, electromagnetic wave noise, particularly the noise produced by signal lines (pattern) which connect devices, can be reduced by sandwiching the signal lines with solid-patch grounds for shielding. "Sandwiching the signal lines with solid-patch grounds" means, for example, a process in which plate-like grounds having certain areas are provided on the first and third layers of a three-layer circuit board, and signal lines are provided in a corresponding area of the second layer, that is, the layer between the first and third layers. The number of layers in this patent application is not literally the number of layers composing a circuit board but the number of conductive pattern planes.
Although solid-patch grounds are able to substantially block propagation of electromagnetic wave noise from the processing circuit to the sensor region, use of the solid-patch ground gives rise to another problem. Because the solid-patch grounds substantially prevent transmission of magnetic flux, the level of magnetic flux becomes lower in a portion close to the solid-patch grounds than in a portion far from the solid-patch grounds. The fluctuation of the level of magnetic flux adversely affects precision of coordinate detection because an electromagnetic induction type coordinate detecting apparatus detects magnetic flux passing through the sensor region and thus generates electric signals.
Further, because the conditions for magnetic flux transmission differ, in principle, from an edge portion to a central portion of the sensor region in an electromagnetic induction type coordinate detecting apparatus, the effective area of the coordinate detection surface inevitably becomes substantially smaller than the entire sensor region. This is particularly disadvantageous for a small-size coordinate detecting apparatus.